Recently, radio communication apparatuses such as a portable telephone, PDA (Personal Digital Assistants) and wireless LAN are daily employed. Since the radio communication apparatuses are designed on a premise that these are carried by users at all times, these apparatuses tend to be miniaturized and formed in a thin structure. With such a tendency, component parts, to be installed in the radio communication apparatuses, have similar tendencies.
In recent radio communication, there are increasing cases where a plurality of frequency bands are utilized. For example, the wireless LAN utilizes wavelengths in 2.4 GHz and 5 GHz bands. For this reason, the antennas for use in the radio communication apparatuses are required to be usable at a plurality of separate frequency bands.
A notebook-sized PC and a portable telephone use an inverted-F antenna, a dielectric antenna or a substrate antenna as built-in antenna. These antennas have features such as omnidirections and high-gains.
However, due to limited conditions in structure, it is hard to minimize the antenna in size and, especially, to form the antenna in a thin structure. When the antenna is installed in the notebook-sized PC, the antenna must be disposed in a limited area, such as a position near a hinge or a frame portion of an LCD (Liquid Crystal Display) because many of the component parts are densely located inside the notebook-sized PC.
Further, the inverted-F antenna of the related art has inherent problems listed below.
As one of the inverted-F antennas of the related art, an antenna which is disclosed in Japanese Patent Provisional Publication No. 2000-68737 has been known. The inverted-F antenna 100 is formed by folding a metallic plate 102 into a substantially U-shaped configuration as shown in FIG. 1. The inverted-F antenna 100 is available to be placed in a narrow space and can be manufactured in a low conducting loss and low cost. An inner conductor 132 of a coaxial cable 130 is electrically connected to a radiating portion 102a of the metallic plate 102. An outer conductor 134 of the coaxial cable 130 is electrically connected to a ground portion 102b of the metallic plate 102.
In order to operate the inverted-F antenna 100 in a plurality of frequency bands, an antenna 110 in which the inverted-F antenna 100 is provided with a parasitic circuit body 104 as shown in FIG. 2 has been known. The antenna 110 comprises the metallic plate 102, the parasitic circuit body 104 and a spacer 106. The parasitic circuit body 104 is disposed on an upper surface of the spacer 106. The spacer 106 is made of dielectric material (non-conductor) and inserted between the radiating portion 102a and the ground portion 102b. Under such a structure, if the inner conductor 132 of the coaxial cable 130 is electrically connected to the radiating portion 102a and the outer conductor 134 of the coaxial cable 130 is electrically connected to the ground portion 102b, the radiating portion 102a and the parasitic circuit body 104 generate a first resonant frequency and a second resonant frequency, respectively.
When the spacer 106 is disposed on the metallic plate 102, it is generally hard to precisely match a distance between the metallic plate 102 and the spacer 106 to a given length. For this reason, the distance between the radiating portion 102a and the parasitic circuit body 104 can not be accurately adjusted into a given length. As a result, the antenna 110 can not obtain accurate resonant frequencies because electrical capacitance between the radiating portion 102a and the parasitic circuit body 104 deviates from a given value. In a case where the resonant frequency generated by the antenna 110 increases, this problem becomes more serious.
An antenna 120 is a modified form of the antenna 110. As shown in FIG. 3, the antenna 120 has the same structure as the antenna 110 except for a shape in which a spacer 122 is different from the spacer 106. The antenna 120 is smaller than the antenna 110 because the spacer 122 is entirely accommodated in a space between the radiating portion 102a and the ground portion 102b of the metallic plate 102. However, the antenna 120 can not obtain accurate resonant frequencies because it is difficult to precisely match the distance between the radiating portion 102a and the parasitic circuit body 104 to a given length.
Also, the above problems arise in a case where a plurality of parasitic circuit bodies are provided to generate a plurality of resonant frequencies.